Projection screen and projection system comprising the same

ABSTRACT

The present invention provides a projection screen capable of sharply displaying an image by minimizing the influence of the loss of light that occurs on a polarized-light selective reflection layer and of providing high image visibility. A projection screen includes: a polarized-light selective reflection layer that selectively reflects a specific polarized-light component; and a substrate that supports the polarized-light selective reflection layer. The polarized-light selective reflection layer includes a plurality of partial selective reflection layers that are laminated to one another, and the partial selective reflection layers have cholesteric liquid crystalline structures, owing to which they selectively reflect a specific polarized-light component. The cholesteric liquid crystalline structure of each partial selective reflection layer comprises a plurality of helical structure parts that are different in direction of helical axis, and, owing to structural non-uniformity in the cholesteric liquid crystalline structure, diffuses light (imaging light) that is selectively reflected. The partial selective reflection layer ( 11   a ) is for selectively reflecting light in the green (G) color wave range, the partial selective reflection layer ( 11   b ) is for selectively reflecting light in the blue (B) color wave range, and the partial selective reflection layer ( 11   c ) is for selectively reflecting light in the red (R) color wave range. These partial selective reflection layers ( 11   a,    11   b  and  11   c ) are successively laminated in this order from the substrate side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection system in which imaginglight emitted from a projector is projected on a projection screen todisplay thereon an image. More particularly, the present inventionrelates to a projection screen capable of sharply displaying an imageand of providing high image visibility, and to a projection systemcomprising such a projection screen.

2. Background Art

A conventional projection system is usually as follows: imaging lightemitted from a projector is projected on a projection screen, andviewers observe the light reflected from the projection screen as animage.

Typical examples of projection screens for use in such conventionalprojection systems include white-colored paper or cloth materials, andplastic films coated with inks that scatter white light. Besides,high-quality projection screens that comprise scattering layerscontaining beads, pearlescent pigments, or the like, capable ofcontrolling the scattering of imaging light, are now commerciallyavailable.

Since projectors have become smaller in size and moderate in price inrecent years, demand for household projectors such as projectors forfamily theaters is growing, and an increasing number of families are nowenjoying projection systems. Household projection systems are oftenplaced in living rooms or the like, which are usually so designed thatenvironmental light such as sunlight and light from lighting fixturescome in abundantly. Therefore, projection screens for use in householdprojection systems are expected to show good image display performanceeven under bright environmental light.

However, the above-described conventional projection screens cannot showgood image display performance under bright environmental light becausethey reflect not only imaging light but also environmental light such assunlight and light from lighting fixtures.

In such a conventional projection system, differences in the intensityof light (imaging light) projected on a projection screen from aprojector cause light and shade to form an image. For example, in thecase where a white image on a black background is projected, theprojected-light-striking part of the projection screen becomes white andthe other part becomes black; thus, differences in brightness betweenwhite and black cause light and shade to form the desired image. In thiscase, in order to attain excellent image display, it is necessary tomake the contrast between the white- and black-indication parts greaterby making the white-indication part lighter and the black-indicationpart darker.

However, since the above-described conventional projection screenreflects both imaging light and environmental light such as sunlight andlight from lighting fixtures without distinction, both the white- andblack-indication parts get lighter, and differences in brightnessbetween white and black become small. For this reason, the conventionalprojection screen cannot satisfactorily provide good image displayunless the influence of environmental light such as sunlight and lightfrom lighting fixtures on the projection screen is suppressed by using ameans for shading a room, or by placing the projection screen in a darkenvironment.

Under these circumstances, studies have been made on projection screenscapable of showing good image display performance even under brightenvironmental light. There have so far been proposed projection screensutilizing, for example, holograms or polarized-light-separating layers(see Japanese Laid-Open Patent Publications No. 107660/1993 (PatentDocument 1) and No. 540445/2002 (Patent Document 2)).

Of these conventional projection screens, those ones using hologramshave the advantage that the white-indication part can be made lighter iftheir light scattering effect is properly controlled, so that they canshow relatively good image display performance even under brightenvironment light. However, holograms have wavelength selectivity but nopolarization selectivity, so that the projection screens using hologramscan display images only with limited sharpness. Moreover, it isdifficult to produce large-sized projection screens by utilizingholograms because of production problems.

On the other hand, on the above-described conventional projectionscreens using polarized-light-separating layers, it is possible to makethe white-indication part lighter and the black-indication part darker.Therefore, these projection screens can sharply display images evenunder bright environmental light as compared with the projection screensusing holograms.

In connection with such projection screens usingpolarized-light-separating layers, we already proposed a projectionscreen comprising a cholesteric liquid crystalline, polarized-lightselective reflection layer, capable of scattering, owing to structuralnon-uniformity in the cholesteric liquid crystalline structure, imaginglight when reflecting it, without lowering image visibility (JapanesePatent Application No. 165687/2003).

SUMMARY OF THE INVENTION

The present invention is to improve the invention of Japanese PatentApplication No. 165687/2003, which is herein incorporated by reference.An object of the present invention is, therefore, to provide aprojection screen capable of sharply displaying an image by minimizingthe influence of the loss of light that occurs on a polarized-lightselective reflection layer, and of providing high image visibility, anda projection system comprising such a projection screen.

The present invention provides, as a first aspect of the invention, aprojection screen for displaying an image by reflecting imaging lightprojected, comprising: a polarized-light selective reflection layer thatselectively reflects a specific polarized-light component and comprisesat least two partial selective reflection layers laminated to eachother, wherein the partial selective reflection layers have structures,owing to which they selectively reflect a specific polarized-lightcomponent; the structures of the partial selective reflection layers aremade different so that the partial selective reflection layersselectively reflect light in different wave ranges for red (R), green(G) and blue (B) colors; and, in the polarized-light selectivereflection layer, the partial selective reflection layer thatselectively reflects light in the green (G) color wave range is situatedon the side opposite to the imaging-light-incident side.

In the above-described first aspect of the invention, it is preferablethat the specific polarized-light component be right- or left-handedcircularly polarized light. The specific polarized-light component mayalso be linearly polarized light of one variation direction.

Further, in the above-described first aspect of the invention, it ispreferable that the projection screen further comprises a diffusingelement that diffuses light that is reflected from the polarized-lightselective reflection layer, or that the polarized-light selectivereflection layer itself has diffusing properties.

Furthermore, in the above-described first aspect of the invention, it ispreferable that the polarized-light selective reflection layer has acholesteric liquid crystalline structure and, owing to structuralnon-uniformity in the cholesteric liquid crystalline structure, diffuseslight that is selectively reflected. In this case, it is preferable thatthe cholesteric liquid crystalline structure of each of the partialselective reflection layers comprises a plurality of helical-structureparts that are different in direction of helical axis.

Furthermore, in the above-described first aspect of the invention, it ispreferable that the projection screen further comprises a substrate thatsupports the polarized-light selective reflection layer comprising thepartial selective reflection layers, situated underneath the partialselective reflection layer that selectively reflects light in the green(G) color wave range. It is herein preferable that the substrate may bean absorptive substrate comprising a light-absorbing layer that absorbslight in the visible region, or a transparent substrate that transmitsat least part of light in the visible region.

Furthermore, in the above-described first aspect of the invention, it isherein preferable that an intermediate layer having adhesion or barrierproperties be provided between each neighboring two of the partialselective reflection layers in the polarized-light selective reflectionlayer.

Furthermore, in the above-described first aspect of the invention, it ispreferable that the projection screen further comprises a functionallayer containing at least one layer selected from the group consistingof a hard coat layer, an anti-glaring layer, an anti-reflection layer,an ultraviolet-light-absorbing layer and an antistatic layer. In thecase where the functional layer is an anti-glaring layer, it ispreferable that the anti-glaring layer comprises a layer with anirregularly roughened surface, isotropic with respect to refractiveindex. For example, a TAC film with a matte surface is conveniently usedas the anti-glaring layer. Instead of providing such an anti-glaringlayer, the imaging-light-incident-side surface of the polarized-lightselective reflection layer may be roughened so that the polarized-lightselective reflection layer shows anti-glaring properties because of theroughened surface.

Furthermore, in the above-described first aspect of the invention, it ispreferable that each of the partial selective reflection layers be madefrom a polymerizable, liquid crystalline material.

The present invention provides, as a second aspect of the invention, aprojection system comprising: a projection screen according to theabove-described first aspect of the invention; and a projector thatprojects imaging light on the projection screen.

According to the present invention, the partial selective reflectionlayers in the polarized-light selective reflection layer are made tohave different structures so that they selectively reflect light indifferent wave ranges for red (R), green (G) and blue (B) colors, and,in the polarized-light selective reflection layer, the partial selectivereflection layer that selectively reflects light in the (G) green colorwave range is positioned on the side opposite to theimaging-light-incident side. Light in the red (R), green (G) and blue(B) color wave ranges, entering from the viewer's side, are reflected atthe corresponding partial selective reflection layers in thepolarized-light selective reflection layer, and return to the viewer'sside. In addition to a selective reflection wave range, color light inthis specific wave range being selectively reflected, each of thepartial selective reflection layers has side bands, light in thesebands, not in the specific wave range of the color light, beingselectively reflected (see FIG. 8 showing a selective reflectionspectrum of light reflected from a cholesteric, polarized-lightselective reflection layer). For this reason, the light reflected fromone partial selective reflection layer toward the viewer's side ispartly reflected at another partial selective reflection layer situatedon the viewer's side of the former partial selective reflection layerbecause of the side bands which the latter partial selective reflectionlayer has. A part of this reflected light becomes stray light, whichbecomes the cause of the loss of light. In the present invention, thepartial selective reflection layer that selectively reflects light inthe wave range for green (G) color whose visibility is high is providedon the side apart from the viewer's side as compared with the partialselective reflection layers that selectively reflect light in the waveranges for red (R) and blue (B) colors whose visibility is low.Therefore, the loss of light in the wave ranges for red (R) and blue (B)colors whose visibility is low is minimized, and light in the red (R),green (G) and blue (B) color wave ranges can be reflected so that thereflected light is excellent in balance from the viewpoint ofvisibility. The projection screen can thus sharply display an image.

Further, according to the present invention, the partial selectivereflection layers in the polarized-light selective reflection layerselectively reflect only a specific polarized-light component (e.g.,right-handed circularly polarized light), so that the polarized-lightselective reflection layer can be made to reflect only approximately 50%of the unpolarized environmental light such as sunlight and light fromlighting fixtures that are incident on this layer. For this reason,while maintaining the brightness of the light-indication part such as awhite-indication part, it is possible to lower the brightness of thedark-indication part such as a black-indication part to nearly half,thereby obtaining nearly twice-enhanced image contrast. In this case, ifthe imaging light to be projected is made to mainly contain apolarized-light component that is identical with the polarized-lightcomponent which the partial selective reflection layers in thepolarized-light selective reflection layer selectively reflect (e.g.,right-handed circularly polarized light), the partial selectivereflection layers in the polarized-light selective reflection layer canreflect nearly 100% of the imaging light projected, that is, thepolarized-light selective reflection layer can efficiently reflect theimaging light.

Furthermore, according to the present invention, if each of the partialselective reflection layers in the polarized-light selective reflectionlayer is made to have a cholesteric liquid crystalline structure, andthis cholesteric liquid crystalline structure is made structurallynon-uniform (for example, the helical-structure parts of the cholestericliquid crystalline structure have helical axes extending in differentdirections), the polarized-light selective reflection layer reflectsimaging light not by specular reflection but by diffuse reflection, andthe reflected light can thus be well recognized as an image. At thistime, owing to structural non-uniformity in the cholesteric liquidcrystalline structures, the partial selective reflection layers in thepolarized-light selective reflection layer diffuse light that isselectively reflected, so that they can reflect a specificpolarized-light component while diffusing it, and, at the same time,transmit the other light components without diffusing them. For thisreason, the environmental light and imaging light that pass through thepartial selective reflection layers in the polarized-light selectivereflection layer do not undergo so-called depolarization, that is, thedisturbance of the state of polarization, and it is thus possible toimprove image visibility while maintaining thepolarized-light-separating property inherent in the polarized-lightselective reflection layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a diagrammatic sectional view showing a projection screenaccording to an embodiment of the present invention;

FIGS. 2A and 2B are illustrations for explaining the state oforientation of and optical function of the polarized-light selectivereflection layer in the projection screen shown in FIG. 1;

FIG. 3 is a diagrammatic sectional view showing a modification of theprojection screen shown in FIG. 1;

FIG. 4 is a diagrammatic sectional view showing another modification ofthe projection screen shown in FIG. 1;

FIG. 5 is a diagrammatic sectional view showing a further modificationof the projection screen shown in FIG. 1;

FIGS. 6A and 6B are diagrammatic sectional views showing still furthermodifications of the projection screen shown in FIG. 1;

FIG. 7 is a diagrammatic view showing an example of a projection systemcomprising a projection screen according to an embodiment of the presentinvention; and

FIG. 8 is a view showing a selective reflection spectrum of lightreflected from a cholesteric, polarized-light selective reflection layer(partial selective reflection layer).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

By referring to the accompanying drawings, embodiments of the presentinvention will be described hereinafter.

Projection Screen

First of all, a projection screen according to an embodiment of thepresent invention will be described with reference to FIG. 1.

As shown in FIG. 1, a projection screen 10 according to this embodimentis for displaying an image by reflecting imaging light projected fromthe viewer's side (the upper side of the figure), and comprises apolarized-light selective reflection layer 11 capable of selectivelyreflecting a specific polarized-light component (e.g., right-handedcircularly polarized light), and a substrate 12 that supports thepolarized-light selective reflection layer 11.

Of these component layers, the polarized-light selective reflectionlayer 11 comprises three partial selective reflection layers 11 a, 11 band 11 c that are laminated to one another, and these partial selectivereflection layers 11 a, 11 b and 11 c have cholesteric, liquidcrystalline structures, owing to which they selectively reflect aspecific polarized-light component.

Each partial selective reflection layer 11 a, 11 b or 11 c is made froma cholesteric, liquid crystalline composition, and physically, liquidcrystalline molecules in this layer are aligned in helical fashion inwhich the directors of the liquid crystalline molecules are continuouslyrotated in the direction of the thickness of the layer.

Owing to such a physical alignment of the liquid crystalline molecules,each partial selective reflection layer 11 a, 11 b or 11 c has thepolarized-light-separating property, the property of separating a lightcomponent circularly polarized in one direction from a light componentcircularly polarized in the opposite direction. Namely, each partialselective reflection layer 11 a, 11 b or 11 c converts unpolarized lightthat enters the layer along the helical axis into light in two differentstates of polarization (right-handed circularly polarized light andleft-handed circularly polarized light), and transmits one of theselight and reflects the other. This phenomenon is known as circulardichroism. If the direction of rotation of liquid crystalline molecularhelix is properly selected, a light component circularly polarized inthe same direction as this direction of rotation is selectivelyreflected.

In this case, the scattering of polarized light is maximized at thewavelength λ_(O) given by the following equation (1):λ_(O) =nav·p,  (1)wherein p is the helical pitch in the helical structure consisting ofliquid crystalline molecules (the length of one liquid crystallinemolecular helix), and nav is the mean refractive index on a planeperpendicular to the helical axis.

On the other hand, the width Δλ of the wave range in which thewavelength of light to be reflected falls is given by the followingequation (2):Δλ=Δn·p,  (2)wherein Δn is the value of birefringence.

Namely, as shown in FIG. 1, of the unpolarized light that has enteredthe projection screen 10 from the viewer's side and has been split intoright-handed circularly polarized light 31R and left-handed circularlypolarized light 31L in the selective reflection wave range and intoright-handed circularly polarized light 32R and left-handed circularlypolarized light 32L not in the selective reflection wave range, one ofthe circularly polarized-light components in the wave range (selectivereflection wave range) with the width Δλ, centered at the wavelengthλ_(O) (e.g., right-handed circularly polarized light 31R in theselective reflection wave range) is reflected from the partial selectivereflection layers 11 a, 11 b and 11c as reflected light 33, and theother light (e.g., left-handed circularly polarized light 31L in theselective reflection wave range, and right-handed circularly polarizedlight 32R and left-handed circularly polarized light 32L not in theselective reflection wave range) pass through the partial selectivereflection layers 11 a, 11 b and 11 c, owing to the above-describedpolarized-light-separating property. FIG. 8 is a diagram showing aselective reflection spectrum of such reflected light 33. As shown inFIG. 8, in addition to a selective reflection wave range, color light inthis specific wave range being selectively reflected, each partialselective reflection layer 11 a, 11 b or 11 c has side bands, light inthese bands, not in the specific wave range of the color light, beingselectively reflected.

The cholesteric liquid crystalline structure of each partial selectivereflection layer 11 a, 11 b or 11 c comprises a plurality ofhelical-structure parts 30 that are different in direction of helicalaxis L, as shown in FIG. 2A. Owing to structural non-uniformity in sucha cholesteric liquid crystalline structure, each partial selectivereflection layer 11 a, 11 b or 11 c diffuses light (reflected light 33)that is selectively reflected. The state in which the cholesteric liquidcrystalline structure is structurally non-uniform herein includes: thestate in which the helical-structure parts 30 of the cholesteric liquidcrystalline structure are different in direction of helical axis L; thestate in which at least some of the planes of nematic layers (the planeson which the directors of liquid crystalline molecules point in the sameX-Y direction) are not parallel to the plane of the polarized-lightselective reflection layer 11 (the state in which, in a sectional TEMphoto of a cholesteric liquid crystalline structure specimen that hasbeen stained, continuous curves that appear as light-and-dark patternsare not parallel to the substrate plane); and the state in which finelydivided particles of a cholesteric liquid crystal are dispersed in thecholesteric liquid crystalline structure as a pigment. The “diffusion”that is caused by such structural non-uniformity in the cholestericliquid crystalline structure means that the light (imaging light)reflected from the projection screen 10 is spread or scattered to suchan extent that viewers can recognize the reflected light as an image.

On the contrary, a conventional cholesteric liquid crystalline structureis in the sate of planar orientation, and the helical axes L inhelical-structure parts 30 of the cholesteric liquid crystallinestructure of a polarized-light selective reflection layer 11′ extend inparallel in the direction of the thickness of the layer, as shown inFIG. 2B. Therefore, when the polarized-light selective reflection layer11′ selectively reflects light (reflected light 36), specular reflectionoccurs.

The partial selective reflection layers 11 a, 11 b and 11 c in thepolarized-light selective reflection layer 11 shown in FIG. 1 havecholesteric liquid crystalline structures whose helical pitches are madedifferent so that they can selectively reflect light in different waveranges, included in the visible region (e.g., the wave range of 400 to700 nm), for red (R), green (G) is and blue (B) colors. It is hereinpreferable to use, as the red (R), green (G) and blue (B) color waveranges, the wave ranges of imaging light projected from a projector suchas a liquid crystal projector (e.g., assuming that light is verticallyincident on the partial selective reflection layers 11 a, 11 b and 11 c,the above wave ranges are the selective reflection wave ranges whosecenter wavelengths are 430-460 nm, 540-570 nm, and 580-620 nm).

The wave ranges of 430 to 460 nm, 540 to 570 nm, and 580 to 620 nm thatare used as the red (R), green (G) and blue (B) color wave ranges,respectively, are wave ranges commonly used for color filters, lightsources, or the like for use in displays that produce white color by thethree primary colors. Red (R), green (G) and blue (B) colors are shownas line spectra maximized at specific wavelengths (e.g., in the case ofgreen (G) color, this wavelength is typically 550 nm). However, theseline spectra have certain widths, and moreover, the projected light havewavelengths that vary depending upon the design of the projector, thetype of the light source, and the like. It is, therefore, preferablethat the wave range for each color has a width of 30 to 40 nm. If thered (R), green (G) and blue (B) color wave ranges are set outside theabove-described respective ranges, it is impossible to produce purewhite, and only yellowish or reddish white is obtained.

In this embodiment, in order to minimize the influence of the loss oflight, the partial selective reflection layer 11 a that selectivelyreflects light in the green (G) color wave range, the partial selectivereflection layer 11 b that selectively reflects light in the blue (B)color wave range, and the partial selective reflection layer 11 c thatselectively reflects light in the red (R) color wave range aresuccessively laminated in this order from the substrate 12 side, asshown in FIG. 1. Namely, the partial selective reflection layer 11 athat selectively reflects light in the green (G) color wave range isprovided on the side opposite to the imaging-light-incident side (on thesubstrate 12 side).

It is preferable that each partial selective reflection layer 11 a, 11 bor 11 c be made to have such a thickness that it can reflect nearly 100%of light in a specific state of polarization that is selectivelyreflected (such a thickness that the reflectance is saturated). This isbecause when each partial selective reflection layer 11 a, 11 b or 11 chas a reflectance of less than 100% for a specific polarized-lightcomponent that is selectively reflected (e.g., right-handed circularlypolarized light), it cannot efficiently reflect the imaging light.Although the reflectance of each partial selective reflection layer 11a, 11 b or 11 c depends directly on the number of helical turns, itdepends indirectly on the thickness of each partial selective reflectionlayer 11 a, 11 b or 11 c if the helical pitch is fixed. Specifically, itis said that approximately 4 to 8 helical turns are needed to obtain areflectance of 100%. Therefore, the partial selective reflection layer11 a, 11 b or 11 c that reflects light in the red (R), green (G) or blue(B) color wave range is required to have a thickness of approximately 1to 10 μm although this thickness varies depending on the type of thecomponents of the liquid crystalline composition used for forming thislayer and on the selective reflection wave range of this layer. On theother hand, the partial selective reflection layer 11 a, 11 b or 11 cshould not be made thick limitlessly because if these layers areexcessively thick, it becomes difficult to control the orientation ofthese layers, the layers cannot be made uniform, and the materialsthemselves for these layers absorb light to a greater extent. For thisreason, it is appropriate that each partial selective reflection layerhas a thickness in the above-described range.

Next, explanation for the substrate 12 will be given below.

The substrate 12 is for supporting the polarized-light selectivereflection layer 11, and a material selected from plastic films, metals,paper, cloth, glass, and the like can be used for forming the substrate12.

It is preferable that the substrate 12 comprises a light-absorbing layercapable of absorbing light in the visible region. Specifically, forexample, the substrate 12 may be made of an acrylic sheet or plasticfilm in which a black pigment is incorporated (e.g., a black PET film inwhich carbon is incorporated)(in this case, the substrate 12 itselfserves as a light-absorbing layer (light-absorptive substrate)), or alight-absorbing layer comprising a black pigment or the like may beformed on one surface of a transparent support film such as a plasticfilm. By this, of the unpolarized light entering the projection screen10 from the viewer's side, those light that are inherently not reflectedfrom the projection screen 10 as reflected light (left-handed circularlypolarized light in the selective reflection wave range, and right-handedcircularly polarized light and left-handed circularly polarized lightnot in the selective reflection wave range) and the light that entersthe projection screen 10 from the backside are absorbed by the substrate12. It is, therefore, possible to effectively prevent reflection ofenvironmental light such as sunlight and light from lighting fixturesand production of stray light from imaging light.

Examples of plastic films that can be used as materials for thesubstrate 12 include films of such thermoplastic polymers aspolycarbonate polymers, polyester polymers including polyethyleneterephthalate, polyimide polymers, polysulfone polymers, polyethersulfone polymers, polystyrene polymers, polyolefin polymers includingpolyethylene and polypropylene, polyvinyl alcohol polymers, celluloseacetate polymers, polyvinyl chloride polymers, polyacrylate polymers,and polymethyl methacrylate polymers. Materials for the substrate 12 arenot limited to the above-described polymers, and it is also possible touse such materials as metals, paper, cloth and glass.

Lamination of the partial selective reflection layers 11 a, 11 b and 11c to the substrate 12 is usually conducted by applying cholestericliquid crystalline compositions and then subjecting the applied layersto aligning treatment and curing treatment, as will be described later.

In the above-described lamination process, it is necessary that thecholesteric liquid crystalline structure of each partial selectivereflection layer 11 a, 11 b or 11 c be made not in the state of planarorientation. It is, therefore, preferable to use, as the substrate 12, amaterial whose surface to which the liquid crystalline composition willbe applied has no aligning power. However, even when a material whosesurface to which the liquid crystalline composition will be applied hasaligning power like a stretched film is used, the cholesteric liquidcrystalline structure of each partial selective reflection layer 11 a,11 b or 11 c can be made not in the state of planar orientation if thissurface of the material is subjected in advance to surface treatment,the components of the liquid crystalline composition are properlyselected, or the conditions under which the liquid crystallinecomposition is oriented are controlled.

A process for producing the above-described projection screen 10 will bedescribed hereinafter.

The substrate 12 to which the polarized-light selective reflection layer11 will be laminated is firstly prepared. The surface of the substrate12 to which a liquid crystalline composition will be applied is made tohave no aligning power.

Thereafter, a cholesteric liquid crystalline composition is applied tothe above-prepared substrate 12 and is then subjected to aligningtreatment and curing treatment, whereby a first partial selectivereflection layer 11 a is laminated (fixed) to the substrate 12.

The steps (the steps of application, alignment and curing) forlaminating (fixing) the first partial selective reflection layer 11 a tothe substrate 12 will be described in detail hereinafter.

(Step of Application)

In the step of application, a cholesteric liquid crystalline compositionis applied to the substrate 12 to form thereon a cholesteric liquidcrystal layer. Any of the known methods can be employed to apply theliquid crystalline composition to the substrate 12. Specifically, aroll, gravure, bar, slide, die, slit, or dip coating method can be usedfor this purpose. In the case where a plastic film is used as thesubstrate 12, a film coating method using a so-called roll-to-rollsystem may be used.

For the liquid crystalline composition that is applied to the substrate12, a cholesteric, chiral nematic liquid crystal or a cholesteric liquidcrystal may be used. Although any liquid crystalline material can beused as long as it can develop a cholesteric liquid crystallinestructure, particularly preferable one for obtaining, after curing, anoptically stable, partial selective reflection layer 11 a is apolymerizable liquid crystalline material having polymerizablefunctional groups at both ends of its molecule.

Explanation will be given below with reference to the case where achiral nematic liquid crystal is used for the liquid crystallinecomposition. The chiral nematic liquid crystal is a mixture of apolymerizable, nematic liquid crystalline material and a chiral agent.The chiral agent herein refers to an agent for controlling the helicalpitch in the polymerizable, nematic liquid crystalline material to makethe resulting liquid crystalline composition cholesteric as a whole. Byvarying the chiral power by changing the type of the chiral agent used,or by varying the chiral agent concentration, it is possible to controlthe center wavelength of the selective reflection wave range that variesaccording to the molecular structure of the polymerizable liquidcrystalline material. To the liquid crystalline composition, apolymerization initiator and other proper additives are added.

Examples of polymerizable, nematic liquid crystalline materials includecompounds represented by the following general formulae (1) and (2-i) to(2-xi). These compounds may be used either singly or in combination.

wherein X is an integer of 2 to 5

In the above general formula (1), R¹ and R² independently representhydrogen or methyl group. It is, however, preferable that both R¹ and R²represent hydrogen because a liquid crystalline composition containingsuch a compound shows a liquid crystal phase at temperatures in a widerrange. X is hydrogen, chlorine, bromine, iodine, an alkyl group having 1to 4 carbon atoms, methoxy group, cyano group or nitro group, preferablychlorine or methyl group. Further, in the above general formula (1), aand b that denote the chain lengths of the alkylene groups that serve asspacers between the (meth)acryloyloxy groups on both ends of themolecule and the aromatic rings are independently an integer of 2 to 12,preferably an integer of 4 to 10, more preferably an integer of 6 to 9.Those compounds represented by the general formula (1) in which a=b=0are unstable, easily undergo hydrolysis, and have high crystallinity. Onthe other hand, those compounds represented by the general formula (1)in which a and b are independently an integer of 13 or more have lowisotropic transition temperatures (TI's). Since these compounds showliquid crystal phases at temperatures in narrow ranges, they areundesirable.

Although a polymerizable liquid crystal monomer is, in the abovedescription, used as the polymerizable, nematic liquid crystallinematerial, it is also possible to use, as the polymerizable, nematicliquid crystal material, a polymerizable liquid crystal oligomer orpolymer, a liquid crystal polymer, or the like, properly selected fromconventionally proposed ones.

On the other hand, the chiral agent is a low molecular weight compoundcontaining an optically active site, having usually a molecular weightof not more than 1,500. The chiral agent is used in order to convert thepositive mono-axially-nematic structure of a polymerizable, nematicliquid crystalline material into a helical structure. Any type of lowmolecular weight compounds may be used as the chiral agent as long as itis compatible with the polymerizable, nematic liquid crystallinematerial in the state of solution or melt and can make the liquidcrystalline structure helical without impairing the liquid crystallinityof the material.

The chiral agent that is used for making the structure of the liquidcrystal helical is required to have any type of chirality at least inits molecule. Examples of chiral agents useful herein include thosecompounds having 1, or 2 or more asymmetric carbon atoms, thosecompounds having asymmetric centers on hetero atoms, such as chiralamines or sulfoxides, and those axially chiral compounds havingoptically active sites, such as cumulene and binaphthol. More specificexamples of chiral agents include commercially available chiral nematicliquid crystals such as a chiral dopant liquid crystal “S-811”manufactured by Merck KGaA, Germany.

However, depending on the nature of the chiral agent selected, thefollowing problems can occur: the nematic state of the polymerizable,nematic liquid crystalline material is destroyed, and the polymerizable,nematic liquid crystalline material loses its alignability; and, if thechiral agent is non-polymerizable, the liquid crystalline compositionhas reduced hardenability, and the cured film is poor in reliability.Moreover, the use of a large amount of a chiral agent containing anoptically active site increases the cost of the liquid crystallinecomposition. Therefore, to form a partial selective reflection layerhaving a cholesteric structure with a short helical pitch, it ispreferable to select, as the optically-active-site-containing chiralagent to be incorporated in the liquid crystalline composition, a chiralagent whose helical-structure-developing action is great. Specifically,it is preferable to use one of the compounds represented by thefollowing general formulae (3), (4) and (5), which arelow-molecular-weight compounds whose molecules are axially chiral.

wherein e is an integer of 2 to 5

In the above general formula (3) or (4), R⁴ represents hydrogen ormethyl group; Y is one of the above-enumerated groups (i) to (xxiv),preferably (i), (ii), (iii), (v) or (vii); and c and d that denote thechain lengths of the alkylene groups are independently an integer of 2to 12, preferably an integer of 4 to 10, more preferably an integer of 6to 9. Those compounds represented by the above general formula (3) or(4) in which c or d is 0 or 1 are poor in stability, easily undergohydrolysis, and have high crystallinity. On the other hand, thosecompounds represented by the general formula (3) or (4) in which c or dis 13 or more have low melting points (Tm's). These compounds are lesscompatible with the polymerizable, nematic liquid crystalline material,so that a liquid crystalline composition containing such a compound asthe chiral agent may cause phase separation depending on theconcentration of the compound.

The chiral agent is not necessarily polymerizable. However, if thechiral agent is polymerizable, it is polymerized with the polymerizable,nematic liquid crystalline material to give a stably fixed cholestericstructure. Therefore, from the viewpoint of thermal stability and thelike, it is desirable that the chiral agent be polymerizable. Inparticular, the use of a chiral agent having polymerizable functionalgroups at both ends of its molecule is preferable to obtain a partialselective reflection layer 11 a excellent in heat resistance.

The content of the chiral agent in the liquid crystalline composition isoptimally decided in consideration of the helical-structure-developingability of the chiral agent, the cholesteric liquid crystallinestructure of the resulting partial selective reflection layer 11 a, andso forth. Although the amount of the chiral agent to be added greatlyvaries depending upon the components of the liquid crystallinecomposition, it is from 0.01 to 60 parts by weight, preferably from 0.1to 40 parts by weight, more preferably from 0.5 to 30 parts by weight,most preferably from 1 to 20 parts by weight, for 100 parts by weight ofthe liquid crystalline composition. In the case where the amount of thechiral agent added is smaller than this range, there is a possibilitythat the liquid crystalline composition cannot fully become cholesteric.On the other hand, when the amount of the chiral agent added exceeds theabove-described range, the alignment of liquid crystalline molecules isimpeded, which may adversely affect the liquid crystalline compositionin the course of curing using activating radiation or the like.

Although the liquid crystalline composition can be applied as it is tothe substrate 12, it may be dissolved in a suitable solvent such as anorganic solvent to give an ink in order to make the viscosity of theliquid crystalline composition fit for an applicator or to attainexcellent alignment of liquid crystalline molecules.

Although any solvent can be used for the above purpose as long as it candissolve the above-described polymerizable liquid crystalline material,it is preferable that the solvent does not attack the substrate 12.Specific examples of solvents useful herein include acetone,3-methoxy-butyl acetate, diglyme, cyclohexanone, tetrahydrofuran,toluene, xylene, chlorobenzene, methylene chloride, and methyl ethylketone. The polymerizable liquid crystalline material may be diluted toany degree. However, considering that a liquid crystal itself is amaterial having low solubility and high viscosity, it is preferable todilute the polymerizable liquid crystalline material to such a degreethat the content of the liquid crystalline material in the dilutedsolution is in the order of preferably 5 to 50%, more preferably 10 to30%.

(Step of Alignment)

After applying the liquid crystalline composition to the substrate 12 toform thereon a cholesteric liquid crystal layer in the above-describedstep of application, the cholesteric liquid crystal layer is, in thestep of alignment, held at a predetermined temperature at which thecholesteric liquid crystal layer develops a cholesteric liquidcrystalline structure, thereby aligning liquid crystalline molecules inthe cholesteric liquid crystal layer.

The cholesteric liquid crystalline structure of the partial selectivereflection layer 11 a that should be finally obtained is one not in thestate of planar orientation but in such a state of orientation as isshown in FIG. 2A, in which a plurality of the helical-structure parts 30that are different in direction of helical axis L are present. Even so,it is necessary to conduct alignment treatment. Namely, although it isnot necessary to align, in one direction on the substrate 12, thedirectors of liquid crystalline molecules in the cholesteric liquidcrystalline structure, it is necessary to conduct such alignmenttreatment that a plurality of the helical-structure parts 30 areproduced in the cholesteric liquid crystalline structure.

When the cholesteric liquid crystal layer formed on the substrate 12 isheld at a predetermined temperature at which the cholesteric liquidcrystal layer develops a cholesteric liquid crystalline structure, itshows a liquid crystal phase. At this time, owing to theself-accumulating action of liquid crystalline molecules themselves,continuous rotation of the directors of the liquid crystalline moleculesoccurs in the direction of the thickness of the layer, and ahelical-structure is produced. It is possible to fix this cholestericliquid crystalline structure in a liquid crystal phase state by curingthe cholesteric liquid crystal layer using such a technique as will bedescribed later.

In the case where the liquid crystalline composition applied to thesubstrate 12 contains a solvent, the step of alignment is usuallyconducted along with drying treatment for removing the solvent. Thedrying temperature suitable for removing the solvent is from 40 to 120°C., preferably from 60 to 100° C. Any drying time (heating time) will doas long as a cholesteric liquid crystalline structure is developed andsubstantially all of the solvent is removed. For example, the dryingtime (heating time) is preferably from 15 to 600 seconds, morepreferably from 30 to 180 seconds. After once conducting the dryingtreatment, if it is realized that the liquid crystal layer is not fullyorientated, this layer may be further heated accordingly. In the casewhere a vacuum drying technique is used in this drying treatment, it ispreferable to separately conduct heat treatment in order to align liquidcrystalline molecules.

(Step of Curing)

After aligning liquid crystalline molecules in the cholesteric liquidcrystal layer in the above-described step of alignment, the cholestericliquid crystal layer is cured in the step of curing, thereby fixing thecholesteric liquid crystalline structure that is in the liquid crystalphase state.

To effect the step of curing, it is possible to use: (1) a method inwhich the solvent contained in the liquid crystalline composition isevaporated; (2) a method in which liquid crystalline molecules in theliquid crystalline composition are thermally polymerized; (3) a methodin which liquid crystalline molecules in the liquid crystallinecomposition are polymerized by the application of radiation; or (4) anycombination of these methods.

Of the above methods, the method (1) is suitable for the case where aliquid crystal polymer is used as the polymerizable, nematic liquidcrystalline material that is incorporated in the liquid crystallinecomposition for forming the cholesteric liquid crystal layer. In thismethod, the liquid crystal polymer is dissolved in such a solvent as anorganic solvent, and this solution is applied to the substrate 12. Inthis case, a solidified, cholesteric liquid crystal layer can beobtained by simply removing the solvent by drying. The type of thesolvent, the drying conditions, and so on are the same as those onesthat are used in the aforementioned steps of application and alignment.

The above-described method (2) is for curing the cholesteric liquidcrystal layer by thermally polymerizing liquid crystalline molecules inthe liquid crystalline composition by heating. In this method, the sateof bonding of the liquid crystalline molecules varies according toheating (baking) temperature. Therefore, if the cholesteric liquidcrystal layer is heated non-uniformly, the cured layer cannot be uniformin physical properties such as film hardness and in optical properties.In order to limit variations in film hardness to ±10%, it is preferableto control the heating temperature so that it varies only within ±5%,preferably ±2%.

Any method may be employed to heat the cholesteric liquid crystal layerformed on the substrate 12 as long as it can provide uniformity inheating temperature. The liquid crystal layer may be placed directly ona hot plate and held as it is, or placed indirectly on a hot plate witha thin air layer interposed between the liquid crystal layer and the hotplate and held parallel with the hot plate. Besides, a method using aheater capable of heating the whole of a particular space, such as anoven, may be employed, where the liquid crystal layer is placed in orpassed through such a heater. If a film coater or the like is used, itis preferable to make the drying zone long enough to make the heatingtime sufficiently long.

The heating temperature required is usually as high as 100° C. or more.However, considering the heat resistance of the substrate 12, it ispreferable to limit this temperature to below approximately 150° C. If aspecialized film or the like having significantly high heat resistanceis used as the substrate 12, the heating temperature can be made as highas 150° C. or more.

The above-described method (3) is for curing the cholesteric liquidcrystal layer by photo-polymerizing liquid crystalline molecules in theliquid crystalline composition by the application of radiation. In thismethod, electron beams, ultraviolet rays, or the like suitable for theconditions can be used as the radiation. In general, ultraviolet lightis preferred because of the simplicity of ultraviolet light irradiationsystems. The wavelength of ultraviolet light useful herein is from 250to 400 nm. If ultraviolet light is used, it is preferable to incorporatea photopolymerization initiator in the liquid crystalline composition inadvance.

Examples of photopolymerization initiators that can be incorporated inthe liquid crystalline composition include benzyl (bibenzoyl), benzoinisobutyl ether, benzoin isopropyl ether, benzophenone, benzoyl benzoicacid, benzoyl methylbenzoate, 4-benzoyl-4′-methyldiphenylsulfide,benzylmethyl ketal, dimethylamino-methyl benzoate,2-n-butoxyethyl-4-dimethylaminobenzoate, isoamylp-dimethylaminobenzoate, 3,3′-dimethyl-4-methoxybenzophenone,methyl-benzoyl formate,2-methyl-1-(4-(methylthio)phenyl)-2-morpholinopropan-1-one,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one,1-(4-dodecylphenyl)-2-hydroxy-2-methylpropan-1-one, 1-hydroxycyclo-hexylphenyl ketone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-(4-isopropylphenyl)-2-hydroxy-2-methyl propan-1-one, 2-chlorothioxanthone,2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone,2,4-dimethylthio-xanthone, isopropylthioxanthone, and1-choloro-4-propoxythioxanthone. In addition to photopolymerizationinitiators, sensitizers may be added to the liquid crystallinecomposition unless they hinder the attainment of the object of thepresent invention.

The amount of the photopolymerization initiator to be added to theliquid crystalline composition is from 0.01 to 20% by weight, preferablyfrom 0.1 to 10% by weight, more preferably from 0.5 to 5% by weight, ofthe liquid crystalline composition.

A projection screen 10 comprising a polarized-light selective reflectionlayer 11 composed of a plurality of partial selective reflection layers11 a, 11 b and 11 c can be obtained by repeatedly conducting a series ofthe above-described steps (the steps of application, alignment andcuring) to successively form, on the partial selective reflection layer11 a, the other cholesteric liquid crystal layers (partial selectivereflection layers). Namely, by successively laminating a plurality ofcholesteric liquid crystal layers to the substrate 12 while controllingthe center wavelength of the selective reflection wave range of eachcholesteric, liquid crystalline composition to be applied, it ispossible to obtain a projection screen 10 comprising, as thepolarized-selective reflection layer 11, the partial selectivereflection layer 11 a that selectively reflects light in the green (G)color wave range, the partial selective reflection layer 11 b thatselectively reflects light in the blue (B) color wave range, and thepartial selective reflection layer 11 c that selectively reflects lightin the red (R) color wave range, laminated successively in this orderfrom the substrate 12 side.

In this case, as long as the underlying cholesteric liquid crystal layerhas been solidified, a liquid crystalline composition for forming thesecond or later cholesteric liquid crystal layer can be applied by usingthe same technique as in the formation of the first liquid crystallayer. In this case, continuity is produced between the cholestericliquid crystalline structure (the state of orientation) of the uppercholesteric liquid crystal layer and that of the lower cholestericliquid crystal layer. It is, therefore, unnecessary to provide analignment-controlling layer or the like between these two cholestericliquid crystal layers. However, an intermediate layer such as anadherent layer may be provided between these two cholesteric liquidcrystal layers, as needed.

Thus, according to this embodiment, in the cholesteric is liquidcrystalline structures of the partial selective reflection layers 11 a,11 b and 11 c in the polarized-light selective reflection layer 11, thehelical pitches are made different from one another so that the partialselective reflection layers selectively reflect light in different waveranges for red (R), green (G) and blue (B) colors, and, in thepolarized-light selective reflection layer 11, the partial selectivereflection layer 11 a that selectively reflects light in the green (G)color wave range is provided on the side opposite to theimaging-light-incident side (on the substrate 12 side). Namely, thepartial selective reflection layer 11 a that selectively reflects lightin the wave range for green (G) color whose visibility is high isprovided on the side apart from the viewer's side (on the substrate 12side) as compared with the partial selective reflection layers 11 b and11 c that selectively reflect light in the wave ranges for red (R) andblue (B) colors whose visibility is low. Therefore, although light inthe green (G) color wave range that is reflected at the partialselective reflection layer 11 a and returns to the viewer's side isreflected owing to the side bands of the partial selective reflectionlayers 11 b and 11 c that are situated on the viewer's side of thepartial selective reflection layer 11 a and is thus partially lost,light in the red (R) and blue (B) color wave ranges that are reflectedat the partial selective reflection layers 11 b and 11 c and return tothe viewer's side are not so much lost as the light in the green (G)color wave range. This is because, only the partial selective reflectionlayer 11 c exists on the viewer's side of the partial selectivereflection layer 11 b that selectively reflects light in the blue (B)color wave range, and no partial selective reflection layer exists onthe viewer's side of the partial selective reflection layer 11 c thatselectively reflects light in the red (R) color wave range. For theabove-described reason, the loss of light in the wave ranges for red (R)and blue (B) colors whose visibility is low is minimized, and thepolarized-light selective reflection layer 11 can reflect light in thered (R), green (G) and blue (B) color wave ranges so that the reflectedlight is excellent in balance from the viewpoint of visibility. Theprojection screen can thus sharply display an image.

Further, according to this embodiment, the partial selective reflectionlayers 11 a, 11 b and 11 c in the polarized-light selective reflectionlayer 11 selectively reflect only a specific polarized-light component(e.g., right-handed circularly polarized light) owing to thepolarized-light-separating property of the cholesteric liquidcrystalline structures, so that the polarized-light selective reflectionlayer 11 can be made to reflect only approximately 50% of theunpolarized environmental light such as sunlight and light from lightingfixtures that are incident on this layer. For this reason, whilemaintaining the brightness of the light-indication part such as awhite-indication part, it is possible to lower the brightness of thedark-indication part such as a black-indication part to nearly half,thereby obtaining nearly twice-enhanced image contrast. In this case, ifthe imaging light to be projected is made to mainly contain apolarized-light component that is identical with the polarized-lightcomponent which the partial selective reflection layers 11 a, 11 b and11 c in the polarized-light selective reflection layer 11 selectivelyreflect (e.g., right-handed circularly polarized light), the partialselective reflection layers 11 a, 11 b and 11 c in the polarized-lightselective reflection layer 11 can reflect nearly 100% of the imaginglight projected, that is, the polarized-light selective reflection layer11 can efficiently reflect the imaging light.

Furthermore, according to this embodiment, since each partial selectivereflection layer 11 a, 11 b or 11 c in the polarized-light selectivereflection layer 11 has a structurally non-uniform, cholesteric liquidcrystalline structure in which the helical-structure parts 30 havehelical axes extending in different directions, the polarized-lightselective reflection layer 11 reflects imaging light not by specularreflection but by diffuse reflection, and the reflected light can thusbe well recognized as an image. At this time, owing to structuralnon-uniformity in the cholesteric liquid crystalline structure, eachpartial selective reflection layer 11 a, 11 b or 11 c in thepolarized-light selective reflection layer 11 diffuses light that isselectively reflected, so that it can reflect a specific polarized-lightcomponent (e.g., right-handed circularly polarized light 31R in theselective reflection wave range) while diffusing it, and, at the sametime, transmit the other light components (e.g., left-handed circularlypolarized light 31L in the selective reflection wave range, andright-handed circularly polarized light 32R and left-handed circularlypolarized light 32L not in the selective reflection wave range) withoutdiffusing them. For this reason, the environmental light and imaginglight that pass through the partial selective reflection layers 11 a, 11b and 11 c in the polarized-light selective reflection layer 11 do notundergo so-called depolarization, that is, the disturbance of the stateof polarization, and it is thus possible to improve image visibilitywhile maintaining the polarized-light-separating property inherent inthe polarized-light selective reflection layer 11.

In the above-described embodiment, explanation has been given byreferring to the case where the partial selective reflection layer 11 athat selectively reflects light in the green (G) color wave range, thepartial selective reflection layer 11 b that selectively reflects lightin the blue (B) color wave range, and the partial selective reflectionlayer 11 c that selectively reflects light in the red (R) color waverange are successively laminated in this order from the substrate 12side, as shown in FIG. 1, to form the polarized-light selectivereflection layer 11. However, the order in which the partial selectivereflection layers 11 a, 11 b and 11 c are laminated is not limited tothe above-described one. Namely, as long as the partial selectivereflection layer 11 a that selectively reflects light in the green (G)color wave range is provided on the side opposite to theimaging-light-incident side (on the substrate 12 side), the other twopartial selective reflection layers may be formed in any order; that is,the partial selective reflection layer 11 a that selectively reflectslight in the green (G) color wave range, the partial selectivereflection layer 11 c that selectively reflects light in the red (R)color wave range, and the partial selective reflection layer 11 b thatselectively reflects light in the blue (B) color wave range may besuccessively laminated in this order from the substrate 12 side, asshown in FIG. 3. This is because light in the red (R) and blue (B) colorwave ranges have visibility that is lower than that of light in thegreen (G) color wave range, so that either the partial selectivereflection layer 11 b or 11 c may be provided as the outermost layer onthe viewer's side.

In the above-described embodiment, explanation has been given byreferring to the case where the wave ranges for red (R), green (G) andblue (B) colors are three independent selective reflection wave rangesand where these three selective reflection wave ranges are realized bythe corresponding three partial selective reflection layers 11 a, 11 band 11 c. However, if at least two of the three wave ranges for red (R),green (G) and blue (B) colors are continuous, two partial selectivereflection layers may be used to form the polarized-light selectivereflection layer 11. For example, if the red (R) and green (G) colorwave ranges are included in one selective reflection wave range, thepolarized-light selective reflection layer 11 has such a construction asis shown in FIG. 4. Namely, a partial selective reflection layer 11 dthat selectively reflects light in the red (R) and green (G) color waveranges and a partial selective reflection layer 11 b that selectivelyreflects light in the blue (B) color wave range are laminated in thisorder from the substrate 12 side. Even in this case, the partialselective reflection layer 11 d that selectively reflects light in thegreen (G) color wave range is provided on the side opposite to theimaging-light-incident side (on the substrate 12 side), so that the lossof light in the wave ranges for red (R) and blue (B) colors whosevisibility is low is minimized. Therefore, even such a polarized-lightselective reflection layer 11 can reflect light in the red (R), green(G) and blue (B) color wave ranges so that the reflected light isexcellent in balance from the viewpoint of visibility.

As mentioned in the above embodiment, most preferably used as thepolarized-light selective reflection layer 11 is one comprising thepartial selective reflection layers 11 a, 11 b and 11 c that havestructurally non-uniform, cholesteric liquid crystalline structures. Itis, however, also possible to use the following polarized-lightselective reflection layer.

Namely, in the above-described embodiment, the polarized-light selectivereflection layer 11 comprises, as the partial selective reflectionlayers 11 a, 11 b and 11 c, those ones having cholesteric liquidcrystalline structures, owing to which they selectively reflect eitherright- or left-handed circularly polarized light. It is, however, alsopossible to use, instead of such a polarized-light selective reflectionlayer 11, a layer having a structure that causes the layer toselectively reflect linearly polarized light of one vibration direction,like a linear polarizing element such as a multi-layered reflectivepolarizer.

Further, in the above-described embodiment, those partial selectivereflection layers that diffuse, owing to structural non-uniformity intheir cholesteric liquid crystalline structures, light that isselectively reflected are used as the partial selective reflectionlayers 11 a, 11 b and 11 c in the polarized-light selective reflectionlayer 11. However, as long as the image visibility is ensured, it ispossible to use partial selective reflection layers whose cholestericliquid crystalline structures are in the state of planar orientation andreflect, by specular reflection, light that is selectively reflected.

Furthermore, in the above-described embodiment, each partial selectivereflection layer 11 a, 11 b or 11 c itself in the polarized-lightselective reflection layer 11 has diffusing properties. The presentinvention is not limited to this, and a diffusing element that diffuseslight reflected from the polarized-light selective reflection layer 11may be provided separately from the polarized-light selective reflectionlayer 11.

Furthermore, in the above-described embodiment, a material whose surfaceto which the liquid crystalline composition will be applied has noaligning power is preferably used as the substrate 12. However, even ifa material whose surface to which the liquid crystalline compositionwill be applied has aligning power is used as the substrate 12, it ispossible to control the orientation of the cholesteric liquidcrystalline structure of the partial selective reflection layers 11 a,11 b and 11 c by providing an intermediate layer 13, such as an adherentlayer, between the substrate 12 and the partial selective reflectionlayer 11 a in the polarized-light selective reflection layer 11, asshown in FIG. 5, thereby directing, to a plurality of directions, thedirectors of liquid crystalline molecules constituting the cholestericliquid crystalline structure of the partial selective reflection layer11 a, existing in the vicinity of the intermediate layer 13. Byproviding an intermediate layer 13 such as an adherent layer, it is alsopossible to improve the adhesion between the partial selectivereflection layer 11 a and the substrate 12. For such an intermediatelayer 13, any material can be used as long as it is highly adherent toboth the material for the polarized-light selective reflection layer 11(the partial selective reflection layer 11 a ) and the material for thesubstrate 12, and it is possible to use commercially availablematerials. Specific examples of materials that can be used for theintermediate layer 13 include an adherent-layer-containing PET filmA4100 manufactured by Toyobo Co., Ltd., Japan and adherent materialsAC-X, AC-L and AC-W manufactured by Panack Co., Ltd., Japan. A blackpigment or the like may be incorporated in the intermediate layer 13,thereby using the intermediate layer 13 as a light-absorbing layercapable of absorbing light in the visible region, as in the case of thesubstrate 12. Further, such an intermediate layer 13 as an adherentlayer may be provided between each neighboring two of the partialselective reflection layers 11 a, 11 b and 11 c that are laminated tothe substrate 12.

Furthermore, in the above-described embodiment, a functional layer 19may be provided on the viewer's side surface of the projection screen 10(the polarized-light selective reflection layer 11), as shown in FIG.6A. A variety of layers including a hard coat (HC) layer, ananti-glaring (AG) layer, an anti-reflection (AR) layer, anultraviolet-light-absorbing (UV-absorbing) layer and an antistatic (AS)layer can be used as the functional layer 19.

The hard coat (HC) layer is for protecting the surface of the projectionscreen 10 and preventing it from being scratched or staining. Theanti-glaring (AG) layer is for preventing the projection screen 10 fromglaring. The anti-reflection (AR) layer is for preventing the surface ofthe projection screen 10 from reflecting light. Theultraviolet-light-absorbing (UV-absorbing) layer is for absorbingultraviolet light that is contained in light incident on the projectionscreen 10 and causes yellowing of the liquid crystalline composition.The antistatic (AS) layer is for removing static electricity generatedin the projection screen 10. In the case where the functional layer 19is the antistatic layer, this layer is not necessarily provided on theviewer's side surface of the projection screen 10 and may be provided onthe back surface of the substrate 12. Moreover, carbon particles or thelike may be incorporated in the substrate 12, thereby imparting, to thesubstrate 12 itself, the property of removing static electricity.

The functional layer 19 serving as an anti-glaring layer has theproperty of preventing the surface of the projection screen 10 frommirroring viewers and their surroundings, and is significant for clearimage recognition. A transparent layer with a roughened surface ispreferably used as the anti-glaring layer, and by the use of such alayer, it is possible to effectively prevent mirroring of objects thatoccurs on the surface of the projection screen 10 because of interfacialreflection. Such a transparent layer can be obtained by roughening thesurface of a transparent resin, glass, or the like by such a method assandblasting, transfer of the shape of a mold surface, or chemicaltreatment. The surface of a transparent layer may be roughened eitherirregularly or regularly. To maintain the polarized-light-separatingproperty of the polarized-light is selective layer 11, it is preferablethat the anti-glaring layer be isotropic with respect to refractiveindex. Examples of materials useful for the anti-glaring layer includeglass, resins such as acrylic resins and polyester resins, and TAC(triacetyl cellulose) films with matte surfaces.

To impart anti-glaring properties to the projection screen 10, either ofthe following two methods can be used: the functional layer 19 servingas an anti-glaring layer is formed separately from thepolarized-light-selective reflection layer 11, as shown in FIG. 6A; orthe viewer's side surface of the polarized-light selective reflectionlayer 11 (or the surface of the partial selective reflection layer 11 c,the outermost layer on the viewer's side) is roughened (see referencenumeral 11 e), as shown in FIG. 6B, thereby imparting anti-glaringproperties to the polarized-light selective reflection layer 11 itself.

Although the above embodiment is described by referring to the casewhere the substrate 12 of the projection screen 10 is an absorptivesubstrate containing a light-absorbing layer capable of absorbing lightin the visible region, the substrate 12 may also be a transparentsubstrate capable of transmitting at least part of light in the visibleregion. If a transparent substrate is used, although the advantage ofenhancing image contrast is lost, the projection screen 10 is highlytransparent while not displaying an image and the background can thus beclearly seen through the projection screen 10. Such a projection screen10 can be used in decorative applications; for example, it is fit foruse on a show window. Moreover, by switching the viewing angle accordingto the situation, it is possible to produce a more effectiveeye-catching effect. For this reason, this projection screen 10 canovercome the drawback of conventional information tools using projectorsthat they do not look attractive in a bright environment, and caneffectively be used in such applications as billboards, bulletin boards,and guideboards. Although the transparent substrate is preferably lesshazy, any material selected from acrylic resins, glass, vinyl chlorideresins, etc. may be used as long as it can transmit light. Further, thetransparent substrate is not necessarily colorless, and a colored onemay also be used. Specifically, it is possible to use transparentplastic or glass plates in a color of brown, blue, orange, or the likethat are usually used for partition walls, windows, and so forth.

Furthermore, the above embodiment includes, as mentioned above, anintermediate layer 13 having adhesion properties (an adherent layer) andprovided between the polarized-light selective reflection-layer 11 andthe substrate 12, or between each neighboring two of the partialselective reflection layers 11 a, 11 b and 11 c that constitute thepolarized-light selective reflection layer 11. The intermediate layer 13may have barrier properties in addition to (or in place of) the adhesionproperties. The barrier properties herein mean the following action:when the polarized-light selective reflection layer is laminateddirectly to the substrate, or when one partial selective reflectionlayer is laminated directly to another partial selective reflectionlayer, the constituents of the lower layer are prevented from migratingto (permeating through) the upper layer, or the constituents of theupper layer are prevented from migrating to (permeating through) thelower layer owing to the barrier properties. If substances migratebetween the upper and lower layers, the optical properties (wavelengthselectivity, polarized-light selectivity, diffusing properties, etc.)inherent in the polarized-light selective reflection layer (or thepartial selective reflection layers) that is the upper or lower layerare impaired. However, this can be prevented by the use of theabove-described intermediate layer having barrier properties (barrierlayer). Specifically, for example, in the case where a partial selectivereflection layer is laminated to another partial selective reflectionlayer by applying a cholesteric liquid crystalline composition, there isa possibility that a nematic liquid crystal component contained in theliquid crystalline composition for forming the upper partial selectivereflection layer permeates through the lower partial selectivereflection layer to change (increase) the helical pitch in the lowerpartial selective reflection layer, depending on a kind of cholestericliquid crystalline composition or process conditions. However, even inthis case, if a barrier layer is provided between the lower and upperpartial selective reflection layers, the migration (permeation) of thenematic liquid crystal component does not occur, and the opticalproperties (wavelength selectivity, polarized-light selectivity,diffusing properties, etc.) of the partial selective reflection layersare successfully maintained.

Examples of materials that can be used for forming such a barrier layerinclude modified acrylates, urethane acrylates, polyester acrylates, andepoxy resins. These compounds may be either monofunctional orpolyfunctional and include monomers and oligomers. Specific examples ofthese compounds include ethoxylated trimethylolpropane triacrylate,propoxylated glyceryl triacrylate, pentaerythritol tetraacrylate,ditrimethylolpropane tetraacrylate, dipentaerythritolhydroxypentaacrylate, ethoxylated pentaerythritol tetraacrylate,pentaacrylic ester, pentaerythritol triacrylate, trimethylolpropanetriacrylate, trimethylolpropane PO-modified triacrylate, isocyanuricacid EO-modified triacrylate, trimethylolpropane EO-modifiedtriacrylate, dipentaerythritol penta- or hexa-acrylate, urethaneadducts, aliphatic polyamine epoxy resins, polyaminoamide epoxy resins,aromatic diamine epoxy resins, alicyclic diamine epoxy resins, phenolicepoxy resins, amino epoxy resins, mercaptan epoxy resins, dicyandiamideepoxy resins, and Lewis acid complex epoxy resins.

Projection System

The aforementioned projection screen 10 can be incorporated into aprojection system 20 comprising a projector 21, as shown in FIG. 7.

As shown in FIG. 7, the projection system 20 comprises the projectionscreen 10 and the projector 21 for projecting imaging light on theprojection screen 10.

Of these components, the projector 21 may be of any type, and a CRTprojector, a liquid crystal projector, a DLP (digital light processing)projector, or the like can be used. It is, however, preferable that theimaging light to be projected on the projection screen 10 from theprojector 21 chiefly contains a polarized-light component (e.g.,right-handed circularly polarized light) that is identical with thepolarized-light component which the projection screen 10 selectivelyreflects.

Because of its operating principle, a liquid crystal projector useful asthe projector 21 usually emits light that is polarized substantiallylinearly. In this case, by letting the imaging light emerge from theprojector 21 through a retardation layer 22 or the like, it is possibleto convert the linearly polarized light into circularly polarized lightwithout causing the loss of the amount of light.

A quarter wave plate is preferably used as the retardation layer 22.Specifically, an ideal retardation layer is one capable of causing aphase shift of 137.5 nm for light of 550 nm whose visibility is highest.Further, a wide-wave-range quarter wave plate is more preferable becauseit is applicable to light in all of the red (R), green (G) and blue (B)color wave ranges. It is also possible to use a single retardation layerproduced by controlling the birefringence of a material for this layer,or a retardation layer using a quarter wave plate in combination with ahalf wave plate.

The retardation layer 22 may be externally attached to the exit-aperture of the projector 21, as shown in FIG. 7, or incorporated-intothe inside of the projector 21.

In the case where a CRT or DLP projector is used as the projector 21,since the projector 21 emits unpolarized light, it is necessary to use acircular polarizer composed of a linear polarizer and a retardationlayer in order to convert the unpolarized light into circularlypolarized light. If such a circular polarizer is used, although theamount of light emitted from the projector 21 itself is decreased tohalf, it is possible to effectively prevent the production of straylight or the like from a polarized-light component (e.g., left-handedcircularly polarized light) that is different from the polarized-lightcomponent which the polarized-light selective reflection layer 11 in theprojection screen 10 selectively reflects, thereby enhancing imagecontrast.

A projector 21 usually attains color display utilizing light in the waveranges for red (R), green (G) and blue (B) colors, the three primarycolors. For example, assuming that light enters the projection screen 10vertically to it, the projector 21 projects light in selectivereflection wave ranges whose centers are between 430 nm and 460 nm,between 540 nm and 570 nm, and between 580 nm and 620 nm.

EXAMPLES

The present invention will now be explained more specifically byreferring to the following Example and Comparative Example.

(Example)

A first cholesteric liquid crystal solution having a selectivereflection wave range with a center wavelength of 550 nm was prepared bydissolving, in cyclohexanone, a monomer-containing liquid crystalconsisting of a main component (95% by weight), an ultraviolet-curingnematic liquid crystal, and a polymerizable chiral agent (5% by weight).A liquid crystal containing a compound represented by the above chemicalformula (2-xi) was used as the nematic liquid crystal. A compoundrepresented by the above chemical formula (5) was used as thepolymerizable chiral agent. To the first cholesteric liquid crystalsolution was added 5% by weight of an acetophenone photopolymerizationinitiator available from Ciba Specialty Chemicals K. K., Japan.

By a bar coating method, the above-prepared first cholesteric liquidcrystal solution was applied to a black-colored acrylic sheet with asurface area of 200 mm□ (200 mm×200 mm).

This acrylic sheet was heated in an oven at 80° C. for 90 seconds,thereby conducting aligning treatment (drying treatment). Thus, acholesteric liquid crystal layer containing no solvent was obtained.

Thereafter, 10 mW/cm² of ultraviolet light with a wavelength of 365 nmwas applied to this cholesteric liquid crystal layer in an atmosphere ofnitrogen for 1 minute to cure the cholesteric liquid crystal layer,thereby obtaining a first partial selective reflection layer having aselective reflection wave range whose center wavelength was 550 nm.

Similarly, a second cholesteric liquid crystal solution was applieddirectly to the first partial selective reflection layer and thensubjected to aligning treatment (drying treatment) and curing treatment.Thus, a second partial selective reflection layer having a selectivereflection wave range with a center wavelength of 450 nm was obtained.The procedure used for preparing the second liquid crystal solution wasthe same as the procedure used for preparing the first liquid crystalsolution, provided that the nematic liquid crystal and the chiral agentwere mixed in such a proportion that the resulting layer had a selectivereflection wave range with a center wavelength of 450 nm.

Similarly, a third cholesteric liquid crystal solution was applieddirectly to the second partial selective reflection layer and thensubjected to aligning treatment (drying treatment) and curing treatment.Thus, a third partial selective reflection layer having a selectivereflection wave range with a center wavelength of 600 nm was obtained.The procedure used for preparing the third liquid crystal solution wasthe same as the procedure used for preparing the first liquid crystalsolution, provided that the nematic liquid crystal and the chiral agentwere mixed in such a proportion that the resulting layer had a selectivereflection wave range with a center wavelength of 600 nm.

Thus, there was obtained a projection screen comprising apolarized-light selective reflection layer composed of the first partialselective reflection layer capable of selectively reflecting light inthe green (G) color wave range (light in a selective reflection waverange with a center wavelength of 550 nm), the second partial selectivereflection layer capable of selectively reflecting light in the blue (B)color wave range (light in a selective reflection wave range with acenter wavelength of 450 nm), and the third partial selective reflectionlayer capable of selectively reflecting light in the red (R) color waverange (light in a selective reflection wave range with a centerwavelength of 600 nm), successively laminated in this order from thesubstrate side. The thickness of the first partial selective reflectionlayer was made 4 μm, that of the second partial selective reflectionlayer was made 3 μm, and that of the third partial selective reflectionlayer was made 5 μm. These partial selective reflection layersconstituting the polarized-light selective reflection layer in theprojection screen had cholesteric liquid crystalline structures thatwere not in the state of planar orientation.

(Comparative Example)

A projection screen was produced in the same manner as in the aboveExample, provided that the first, second and third cholesteric liquidcrystal solutions were applied in the order different from that inExample. Thus, there was obtained a projection screen comprising apolarized-light selective reflection layer composed of the first partialselective reflection layer capable of selectively reflecting light inthe blue (B) color wave range (light in a selective reflection waverange with a center wavelength of 450 nm), the second partial selectivereflection layer capable of selectively reflecting light in the green(G) color wave range (light in a selective reflection wave range with acenter wavelength of 550 nm), and the third partial selective reflectionlayer capable of selectively reflecting light in the red (R) color waverange (light in a selective reflection wave range with a centerwavelength of 600 nm), successively laminated in this order from thesubstrate side.

(Results of Evaluation)

Imaging light emitted from a projector was projected on the projectionscreen of Example and that of Comparative Example, and the contrastvalues were determined. In this measurement, a liquid crystal projector(“ELP-52” manufactured by Epson Co., Ltd., Japan) was used as theprojector.

In order to convert the imaging light emitted from the projector intocircularly polarized light, a circular polarizer was placed on the exitaperture of the projector. A fluorescent lamp (emitting unpolarizedlight) fixed to the ceiling was used to illuminate the room in which theprojector and each projection screen were placed, where the projectionscreen and the fluorescent lamp were arranged so that the light from thefluorescent light directly entered the projection screen at an angle ofapproximately 50°. The illumination intensity on the projection screenright under the fluorescent lamp, measured by an illuminometer (adigital illuminometer “510-02” manufactured by Yokogawa M & C Co., Ltd.,Japan), was 200 lx.

The projection screen was set vertically to the floor. The projector wasplaced at such a point that the horizontal distance (in parallel withthe floor) between the projector and the projection screen wasapproximately 2.5 m.

Imaging light (a still image composed of white and black areas) wasprojected on the projection screen from the projector, and the imagecontrast was determined. Specifically, the luminance of the white areaand that of the black area in the center of the projection screen weremeasured by a luminance meter “BM-8” manufactured by Topcon Corp.,Japan, and the ratio between these two luminances was obtained as theimage contrast [contrast=(luminance of white area)÷(luminance of blackarea)].

The contrast values of the images projected on the projection screen ofExample and that of Comparative Example are shown in Table 1. As isclear from Table 1, the images displayed on the projection screen ofExample and that of Comparative Example were found to havesatisfactorily high contrast. TABLE 1 Sample Example Comparative ExampleContrast 30 30

Further, the images on these projection screens were visually observed.They were found excellent.

However, the color of the image displayed on the projection screen ofExample was found more vivid than that of the image displayed on theprojection screen of Comparative Example, when the imaging light (astill image of a red-, green- and blue-colored pattern) projected oneach projection screen from the projector was visually observedslantingly against the imaging light incident on the projection screen(at an angle of 15° or more with the normal to the screen plane, wherethe angle of the normal is 0°).

1. A projection screen for displaying an image by reflecting imaginglight projected, comprising: a polarized-light selective reflectionlayer that selectively reflects a specific polarized-light component andcomprises at least two partial selective reflection layers laminated toeach other, wherein the partial selective reflection layers havestructures, owing to which they selectively reflect a specificpolarized-light component; the structures of the partial selectivereflection layers are made different so that the partial selectivereflection layers selectively reflect light in different wave ranges forred (R), green (G) and blue (B) colors; and, in the polarized-lightselective reflection layer, the partial selective reflection layer thatselectively reflects light in the green (G) color wave range is situatedon the side opposite to the imaging-light-incident side.
 2. Theprojection screen according to claim 1, wherein the specificpolarized-light component is right- or left-handed circularly polarizedlight.
 3. The projection screen according to claim 1, wherein thespecific polarized-light component is linearly polarized light of onevibration direction.
 4. The projection screen according to claim 1,further comprising a diffusing element that diffuses light that isreflected from the partial selective reflection layers in thepolarized-light selective reflection layer.
 5. The projection screenaccording to claim 1, wherein the partial selective reflection layersthemselves in the polarized-light selective reflection layer havediffusing properties.
 6. The projection screen according to claim 5,wherein the partial selective reflection layers in the polarized-lightselective reflection layer have cholesteric liquid crystallinestructures and, owing to structural non-uniformity in the cholestericliquid crystalline structures, diffuse light that is selectivelyreflected.
 7. The projection screen according to claim 6, wherein thecholesteric liquid crystalline structure of each of the partialselective reflection layers comprises a plurality of helical-structureparts that are different in direction of helical axis.
 8. The projectionscreen according to claim 1, further comprising a substrate thatsupports the polarized-light selective reflection layer comprising thepartial selective reflection layers, situated underneath the partialselective reflection layer that selectively reflects light in the green(G) color wave range.
 9. The projection screen according to claim 8,wherein the substrate is an absorptive substrate comprising alight-absorbing layer that absorbs light in a visible region.
 10. Theprojection screen according to claim 8, wherein the substrate is atransparent substrate that transmits at least part of light in a visibleregion.
 11. The projection screen according to claim 1, furthercomprising an intermediate layer having adhesion properties, providedbetween each neighboring two of the partial selective reflection layersin the polarized-light selective reflection layer.
 12. The projectionscreen according to any of claim 1, further comprising an intermediatelayer having barrier properties, provided between each neighboring twoof the partial selective reflection layers in the polarized-lightselective reflection layer.
 13. The projection screen according to claim1, further comprising a functional layer containing at least one layerselected from a group consisting of a hard coat layer, an anti-glaringlayer, an anti-reflection layer, an ultraviolet-light-absorbing layerand an antistatic layer.
 14. The projection screen according to claim13, wherein the functional layer is an anti-glaring layer that comprisesa layer with an irregularly roughened surface, isotropic with respect torefractive index.
 15. The projection screen according to claim 14,wherein the anti-glaring layer is a TAC film with a matte surface. 16.The projection screen according to claim 1, wherein theimaging-light-incident-side surface of the polarized-light selectivereflection layer is roughened, and the polarized-light selectivereflection layer shows anti-glaring properties because of the roughenedsurface.
 17. The projection screen according to claim 1, wherein each ofthe partial selective reflection layers is made from a polymerizable,liquid crystalline material.
 18. A projection system comprising: aprojection screen according to claim 1; and a projector that projectsimaging light on the projection screen.